Foreign Minister Mevlut Cavusoglu says Turkey may buy U.S. Patriot missile systems if conditions are right, but insists such a deal would be impossible if Washington forces Ankara to cancel its agreement to purchase S-400 antiaircraft missiles from Russia.

In an interview with Turkey’s NTV on January 10, Cavusoglu said his NATO-member state will not accept the United States imposing conditions in regard to its deal to buy the Russian-made surface-to-air defense systems.

Meanwhile, in another sign of deteriorating relations between Ankara and Washington, Cavusoglu said a military operation Turkey was planning against U.S.-backed Kurdish militia in northern Syria did not depend on a withdrawal of U.S. troops from Syria.

Cavusoglu told NTV it was not realistic to expect the United States to collect all of the weapons it had supplied to Syrian Kurdish fighters who are viewed by Ankara as terrorists.

U.S. President Donald Trump’s announcement in late December that he planned to withdraw some 2,000 U.S. troops from Syria stunned U.S. allies and led to the resignation of U.S. Defense Secretary Jim Mattis.

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But U.S. national security adviser John Bolton told Turkish officials in Ankara on January 8 that Turkey’s assurance it won’t attack the U.S.-backed Kurdish fighters was a “condition” for the withdrawal.

Turkish President Recep Tayyip Erdogan accused Bolton of making “a very serious mistake” with the demand.

“We cannot make any concessions in this regard,” said Erdogan, who vowed that “those involved in a terror corridor” in Syria “will receive the necessary punishment.”

The U.S.-backed Syrian Kurdish People’s Protection Units form the backbone of the opposition Syrian Democratic Forces and have been fighting alongside U.S. troops against Islamic State militants in northeastern Turkey.

But Ankara insists those Syrian Kurdish fighters are linked to the Kurdish Workers Party (PKK), a group that is banned in Turkey and has been considered a terrorist group by the United States since 1997.